More travel than...? A monster truck? Giant's recommendations? The 05 Reign1 came stock with a 150mm Talas 36 (in NZ), the 06 Reign2 came with a 130mm Revelation. Identical frames...

As it was explained to me by a Giant dealer, if you change the the fork to something with different travel than what was provided on the stock, factory built bike.
I have a 100/120 on my Iguana, which came stock with an 80, which voided my warranty.

So, I would assume that anything that deviates from the factory build, plus or minus, would void the warranty.

I was only told that it couldn't be above the travel... Even if it is a fork that has options to tune it to 100, 120, 130mm and you only run it at 100 its no good. A rep at Giant (called their number on website) told me that the travel they provide stock is the most travel that they allow under warranty.

Like with any company you would have to be mindful of getting caught out on a technicality if it came to the crunch (or crack LOL).

It seems rather stupid that my Trance could come with a Tora which is easily capable of going to 130mm travel (by removing a spacer during service) and yet doing so would void the warranty. The other trance models at the time came with Reba, which can be changed to 115mm. If they want to say something like that then the company should be more careful in their spec parts. My bike was an '06 & it felt really good with a 130mm travel fork on the front, and actually quite incidentally it would have had the same geometry angles at that setting as the new trance X (simply without the added benefit of more rear travel).

Another example is the STP, the top model here in Australia comes with a Pike, U-Turn from 95-140mm. The model I brought (STP 0) came with a 100mm Argyle which I believe can be adjusted to 80mm with the included spare spacers inserted at service. I did replace the Argyle with a Pike. Now common sense says that it's perfectly fine because the model up is exactly the same frame and comes with that fork. But technically speaking if I broke my bike they could rightly say I had modified bike from it's original spec and therefore the warranty is void.

So I'd say use your common sense! I've got a Giant Alias (it's like the model above the RIncorn / Yukon). Anyway the Tora is on that now but back down to 100mm (which is the travel of the Dart that the bike came with). Now on that bike I think ideally I probably wouln't go past 115mm travel as after that it just doesn't feel right, and common sense says an entry level XC HT probably shouldn't have much more variation than that from it's original spec and intention.

I think that +/- of up to 20mm is considered generally a pretty safe margin to play with, but of course at your own risk or otherwise the manufacturers discretion when it comes warranty time.

Just like Ducktape said it all has to do with common sense. Giant's just covering themselves by instituting (and sticking with) this policy, and it makes sense. If they were to allow one person to send back a frame where they were running 15mm more than stock then by the same token they would need to allow the tool who puts an RS Totem on an NRS to send their bike back for a structural failure as well. A policy that isn't backed up is worth nothing.

With this said like William42 stated just go to the dealer if your're thinking about a different fork. They should be able to direct you on the right path that won't void your warranty. If you decide that you really want to go bigger then remember Stoner's advice.

put whatever you want on there, it will not void the warranty, only thing that will void the warranty is a dual crown on a non glory frame, i work at a Giant dealer, have a reign x, called them up asked for warranty, they said i can put as tall a single crown on there i want, just not a duel. there are a tone of guys running 66's on reigns and reign x's and it is fine. call giant, dont listen to the shop, 1 800 us giant